Aqueous-based polymer dispersions find use in a wide variety of fields such as adhesives, binders, sizing agents, sealants, primers, inks, paints, seed or grain coating systems, and paper coatings. Such dispersions also find use in pharmaceutical industries for drug delivery, in medical testing kits, or as additives in cosmetics. A major use of such dispersions has been to form thin barrier coatings on paper, paperboard packaging, various plastic films, and containers. However, the synthetic polymers typically used to form such dispersions and coatings are typically sourced from petroleum. This can sometimes make the polymers difficult to recycle and problematic in landfills where biodegradability is required.
To address these problems, the art has turned to biodegradable polymers such as, for example, polylactic acid (PLA), polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), polyhydroxyvalerate (PHV), polyhydroxyhexanoate (PHH), copolymers of glycolic and lactic acid (PLGA), and polycaprolactones. Initial efforts at making coatings of such biodegradable polymers involved melting such polymers and then extruding them into thin films. However, this required working the polymers at high temperatures which could cause premature degradation of the polymers and employed extrusion and drawing techniques that were equipment and energy intensive.
Later efforts involved dissolving the biodegradable polymers in volatile organic solvents, coating the dissolved materials onto substrates, and then removing the solvents. These later efforts generated a different set of problems in the handling and disposal of such volatile compounds. These problems included toxicity, flammability, and environmental pollution. In addition, such films can also leave some residual solvent in the coating which can adversely affect organoleptic properties.
More recently, efforts have been made to provide processes of making aqueous dispersions of biodegradable polymers which do not use volatile organic solvents. Doi et al., U.S. Pat. No. 6,716,911, teach making an aqueous biodegradable polymer dispersion by melt kneading the polyester and an aqueous emulsifier solution in an extruder. However, the pH of the dispersion must be controlled within relatively narrow limits to avoid hydrolysis of the polyester. Tanaka et al., U.S. Pat. No. 2015/0005174, teach a novel polylactic polyester resin that is said to exhibit superior water dispersibility so that it can be formulated into an aqueous dispersion without the need for emulsifiers or organic solvents. However, the dispersion is specific to a narrow class of defined polymer structures. Whitehouse, U.S. Pat. No. 9,085,688, teaches a process for producing aqueous dispersions of hydroxyalkanoate polymers or copolymers using a heated twin screw extruder. Water and surfactants are added to the hot polymer melt to form the dispersion. Again, however, the dispersion is limited to a narrow class of a specific polymer.
Accordingly, there remains a need in this art for processes which are effective to provide for broader-based aqueous dispersions which are both substantially free from organic solvents and hydrolytically stable over a wide pH range.